Cation–π interaction: a case for macrocycle–cation π-interaction by its ureidoarene counteranion

We report the solid state structures of homocomplex [1·K]+[I]− and of heterocomplexes [1·K]+[3·I]− and [1·K]+[4·I]. In the [1·K]+[I]− complex the apical position of the K+ in the macrocycle is occupied by a –CH2– moiety of a neighboring 18-crown-6, consistent with a close contact and corresponding to ‘agostic’ interactions between K+ and –CH2– moieties. In the [1·K]+[3·I]− complex the second apical site of the cation is coordinated by a bridging water molecule which is simultaneously H-bonded to both the phenol hydroxyl and the iodide anion. In the [1·K]+[4·I]− complex the second apical site of the cation is occupied by the indole moiety. Moreover, the new heterocomplex system [1·K]+[4·I]− presented here, despite the multiple possibilities of K+–π contacts with the sterically available phenyl, phenyl-indole and pyrrole-indole rings of 4, shows that the pyrrolo C2C3 double bond is a versatile π-donor. 1H NMR results led us to conclude that the complexes adopt similar conformations in solution to those observed in the solid state.